1. Field of the Invention
This invention relates to non-invasive oximetry of the pulse type in which light, which has irradiated a volume of arterial blood within a subject, is sensed in order to determine oxygen saturation of the subject's blood. The light which contacts the subject, depending on its spectral content, is variously absorbed, reflected, scattered and transmitted by the blood and other tissue of the subject, before detection. The intensity and the change of intensity of either the reflected, scattered, or the transmitted light are used to determine oxygen saturation of the subject's blood. Such change is essentially due to the arterial pulse, which causes the volume of irradiated blood to vary in accordance with the arterial pulse.
2. Statement of the Prior Art
U.S. Pat. No. 3,847,483, issued Nov. 12, 1974 to Shaw et al, describes and claims an invasive oximetry method and apparatus wherein blood optical density is determined from measures of reflected intensity of red and infrared light transmitted to and from the blood. Both time and frequency multiplexing are used to separate red intensity from infrared intensity.
U.S. Pat. No. 4,086,915, issued May 2, 1978 to Kofsky et al, describes and claims a non-invasive oximetry method and apparatus wherein change of blood optical density, due to the arterial pulse, is determined from measures of change in transmitted or reflected intensity of red and infrared light incident on ear or other blood-perfused tissue of a living subject. Time multiplexing is used for separating change in red intensity from change in infrared intensity.
In the same vein are Hamaguri U.S. Pat. No. 4,266,554, May 12, 1981; Wilber U.S. Pat. No. 4,407,290, Oct. 4, 1983; New et al U.S. Pat. No. 4,653,498, Mar. 31, 1987; Edgar et al, U.S. Pat. No. 4,714,080, Dec. 22, 1987; and Aoyagi et al, laid-open Japanese Patent Application No. Sho 50/1975-128387. These disclose various circuit realizations and oximetric mathematical exegeses differing in detail from those which Kofsky, et al, supra apply to non-invasive, pulse oximetry.
Again, frequency multiplexing in oximetry is referred to in Yee et al, "A Proposed Miniature Red/Infrared Oximeter Suitable for Mounting on a Catheter Tip", IEEE Trans. Biomed. Eng., Vol. 24, pp. 195-197, March, 1977, who describe invasive and non-invasive oximetric probes, and, as well, Schibli et al, "An Electronic Circuit for Red/Infrared Oximeters", IEEE Trans. Biomed Eng., Vol. BME-25, No. 1, pp. 94-96, January, 1978, describe in vitro oximetry wherein blood optical density is determined from measures of reflected intensity of red and infrared light transmitted to and from extracorporeal blood using a probe of Yee et al, supra. V. M. Krishnan, in a Jun. 6, 1973 thesis for the Master of Science degree in Electrical Engineering, University of Washington, describes details of circuitry relating to frequency multiplexing, which Yee et al include by reference in their paper.
Lastly, Huch et al "LIMITATIONS OF PULSE OXIMETRY", The Lancet, Feb. 13, 1988, pp. 357, 358, presents one recent view of the character of results obtained by current pulse oximetric methods and apparatus.
It is the main object of the present invention to provide an oximeter wherein oxygen saturation measurement is based on non-invasively obtained, frequency-multiplexed, arterial-pulse modulated information contained in red and infrared radiation from blood-perfused tissue.
A particular object of the invention is to provide such oximeter wherein the red and infrared information is obtained from red and infrared light reflected from and scattered in the blood-perfused tissue.